Outdoor bronze statues: analysis of metal and surface samples

Abstract

Results are presented from the analysis of 34 metal samples and 168 surface samples taken before conservation from cast bronze statues in 16 monuments in Ottawa, Canada. Metal samples, analyzed semi-quantitatively using standardless X-ray microanalysis, were predominantly copper with small amounts of tin, zinc and sometimes lead. Surface samples were analyzed by powder X-ray diffraction, X-ray energy spectrometry, Fourier transform infrared spectroscopy and/or gas chromatography-mass spectrometry. Of the 94 crystalline compounds identified in the surface samples, those detected most frequently were quartz, copper sulphate hydroxides (for example, brochantite, antlerite), copper chloride hydroxides (for example, atacamite, para tacamite), cuprite and gypsum. The main organic materials were a drying oil (probably linseed oil), poly(dimethyl siloxane) and waxes. The results presented here demonstrate the importance of two aspects of the conservation process: the need for proper surface cleaning to remove water-soluble compounds so they do not become trapped beneath a coating, and the need to apply and maintain a protective coating to limit contact between acid precipitation and acid-soluble compounds.